1. Field of the Invention
This invention relates to an improved dyestuff-containing microscopic capsule dispersion for record materials, which capsules are prevented from coloration, and more particularly to a dispersion in a liquid medium of microscopic capsules of a hydrophobic solvent solution containing an electron donative dyestuff which capsules are prevented from coloration and adopted to produce record materials such as pressure sensitive recording paper.
2. Description of the Prior Art
As a recording system making use of the color reaction through the mutual contact between a wide variety of electron donative dyestuffs and electron attractive acidic developers, there have been known pressure sensitive recording paper and the like.
The production of such pressure sensitive recording paper has been considerably increased in recent years as carbonless copying paper (i.e., non-carbon paper) with the trend of office work rationalization and the popularization of computers. Its demand is expected to increase still further in the future.
Pressure sensitive recording paper was first rendered marketable upon completion of the microencapsulation technology for a solution containing an electron donative dyestuff, taking the hint from the color reaction between crystal violet lactone (hereinafter, abbreviated as "CVL") and acidic clay. Owing to the subsequent technology improvement in various fields such as dyestuffs, developers, solvents for dyestuffs, microencapsulation technique and coating technique, the quality and performance of pressure sensitive recording paper have been steadily improved.
As electron attractive acidic developers, in addition to acidic clay which has been used from the dawn of pressure sensitive recording paper, other developers have been proposed and actually used, including phenolformaldehyde polymer, metal-modified phenol-formaldehyde polymer, substituted salicylic acids and their multivalent metal salts.
As electron donative dyestuffs, a number of dyestuffs have been proposed including (1) various phthalide dyestuffs led by CVL; (2) various fluoran dyestuffs; (3) various azaphthalide dyestuffs; (4) leucoauramine dyestuffs; (5) phthalan dyestuffs; (6) spiropyran dyestuffs; (7) acylleucophenothiazine dyestuffs; (8) diphenylmethane dyestuffs; and (9) triphenylmethane dyestuffs. In accordance with the development of new developers, besides CVL (phthalide) and benzoylleucomethylene blue (acylleucophenothiazine) that have actually been used from the beginning, varied types of phthalide dyestuffs, fluoran dyestuffs and azaphthalide dyestuffs have been adopted for actual use or are about to be used actually.
These dyestuffs are dissolved in a dyestuff solvent and encapsulated for use in the production of pressure sensitive recording paper. In such microscopic capsules, in place of polychlorinated biphenyls which were employed in the beginning, other hydrophobic solvents of low toxicity and high boiling point have been proposed and actually used including partially hydrogenated terphenyls, alkyldiphenyls, alkylbenzenes, alkylnaphthalenes, diallylalkanes and alkyldiphenylethers.
Regarding the microencapsulation method of the dyestuff-containing solvent, in addition to the microencapsulation making use of the gelatin-type coacervation method which was employed in the initial stage of the microencapsulation technology, a wide variety of microencapsulation techniques which are improved in both quality and applicability and make use of synthetic resin (for example, urea-formaldehyde, melamine-formaldehyde, polyamide and polyurethane resins, etc.) have been proposed. Some of such new microencapsulation techniques have already been employed in actual production.
Pressure sensitive recording paper is featured in that it can promptly produce a color of high intensity upon application of a writing or typing pressure. It is however accompanied by serious drawbacks such that the fastness of produced color marks (namely, light resistant fastness, heat resistant fastness, solvent resistant fastness, etc.) is poor and, upon exposure to light or contact with a polar solvent such as a plasticizer or during its storage at high temperatures, the color marks are susceptible of fading out and become eventually illegible. Thus, remedies for such drawbacks have been strongly waited for.
As a technique to improve the fastness of color marks produced on the aforementioned pressure sensitive recording paper, it has been proposed to use methine dyestuffs led by triphenyl methane dyestuffs (see, Japanese Patent Publication No. 16052/1971; and Japanese Patent Laid-open No. 20808/1975).
Although pressure sensitive recording paper, which makes use of a methine dyestuff led by a triphenyl methane dyestuff and an acidic color developer (for example, a clay-type color developer such as acidic clay or phenol condensate-type color developer), is slow in color-producing speed, it has a merit that it can provide color marks having far better fastness than color marks obtained from the combination of a phthalide dyestuff such as CVL or fluoran dyestuff and an acidic color developer.
However, such methine dyestuffs are accompanied by such drawbacks that (a) many methine dyestuffs are unstable during their storage and tend to develop colors due to photochemical reactions; (b) when they are dissolved in a hydrophobic solvent and converted to microscopic capsule dispersions in accordance with various encapsulation methods such as the complex coacervation method relying upon a gelatin-gum arabic system and the in-situ polymerization method of polyurea films to produce pressure sensitive copying paper, many methine dyestuffs are badly colored; and (c) pressure sensitive recording paper obtained by coating such microscopic capsule dispersions on substrate sheets are colored and give a visual impression different from that available from ordinary paper. Due to such drawbacks, it has been considered difficult to carry them out to practical use.